1. Field of the Invention
The present invention relates to a zoom lens, and more specifically, it relates to a small high-optical-performance zoom lens used in small image pickup apparatuses (for example, digital still cameras or digital video cameras).
2. Description of the Related Art
A zoom lens that includes three lens units of negative, positive, and positive refractive powers in order from the object side to the image side and that performs zooming by moving these lens units is widely used as a zoom lens for small digital cameras because the number of lenses is comparatively small and the size can be easily reduced (U.S. Pat. No. 6,618,210, Japanese Patent Laid-Open No. 11-194274, Japanese Patent Laid-Open No. 4-114116, and U.S. Pat. No. 6,304,389).
In U.S. Pat. No. 6,618,210, a zoom ratio of about three times is achieved with seven to ten lenses by specifying the refractive power and total magnification of the second and third lens units.
In Japanese Patent Laid-Open No. 11-194274, a compact zoom lens having a zoom ratio of about three times is achieved with seven lenses by specifying the locus of zooming and the refractive power of each lens unit.
In Japanese Patent Laid-Open No. 4-114116, a small zoom lens having a zoom ratio of about six times is achieved with seven to eight lenses by specifying the refractive power of the first and second lens units and the lens configuration.
In U.S. Pat. No. 6,304,389, a zoom lens having a zoom ratio of about five times and a field angle of 60° or more at the wide-angle end is achieved with nine to ten lenses by not moving the third lens unit during zooming from the wide-angle end to the telephoto end and appropriately setting the refractive power of each lens unit and the distances between the lens units.
However, these zoom lenses have a zoom ratio of about three to six times and do not meet the requirements of a small image pickup apparatus having a high zoom ratio of more than six times.
In recent years, zoom lenses for small digital cameras or digital video cameras have been required to be small and to have a high zoom ratio. In general, in order to increase the zoom ratio, it is necessary to increase the refractive power of each lens unit and to increase the moving distance during zooming. However, increasing the refractive power of each lens unit makes it difficult to obtain excellent optical performance throughout the zoom range with a small number of lenses. In order to obtain excellent optical performance, it is necessary to increase the number of lenses. This makes it difficult to reduce the size of the whole lens system. Increasing the moving distance during zooming increases the overall length of the zoom lens and makes it difficult to reduce the size of the whole lens system.